Kinases regulate many different processes such as cell proliferation, differentiation, and cell signaling by adding phosphate groups to proteins. Uncontrolled signaling has been implicated in a variety of disease conditions such as inflammation, cancer, arteriosclerosis, and psoriasis. Reversible protein phosphorylation is the main strategy for controlling activities of eukaryotic cells. It is estimated that more than 1000 of the 10,000 proteins active in a typical mammalian cell are phosphorylated. The high energy phosphate which drives phosphorylation is generally transferred from adenosine triphosphate molecules (ATP) to a particular protein by protein kinases and removed from that protein by protein phosphatases. Phosphorylation occurs in response to extracellular signals such as hormones, neurotransmitters, growth and differentiation factors, etc. cell cycle checkpoints, and environmental or nutritional stresses. An appropriate protein kinase can activate a metabolic enzyme, regulatory protein, receptor, cytoskeletal protein, ion channel or pump, or transcription factor.
Kinases comprise the largest known protein group, a superfamily of enzymes with widely varied functions and specificities. They are usually named after their substrate their regulatory molecules, or some aspect of a mutant phenotype. With regard to substrates, the protein kinases may be roughly divided into two groups; those that phosphorylate tyrosine residues (protein tyrosine kinases, PTK) and those that phosphorylate serine or threonine i residues (serine/threonine kinases, STK). A few protein kinases have dual specificity and phosphorylate serine, threonine and tyrosine residues. Almost all kinases contain a conserved 250-300 amino acid catalytic domain. The N-terminal domain, which contains subdomains I-IV, generally folds into a two-lobed structure which binds and orients the ATP (or GTP) donor molecule. The larger C terminal lobe, which contains subdomains VI-XI, binds the protein substrate and carries out the transfer of the gamma phosphate from ATP to the hydroxyl group of a serine, threonine, or tyrosine residue. Subdomain V spans the two lobes. The kinases may be categorized into families by the different amino acid sequences (generally between 5 and 100 residues) located on either side of, or inserted into loops of, the kinase domain. These added amino acid sequences allow the regulation of each kinase as it recognizes and interacts with its target protein. The primary structure of the kinase domain is conserved and can be further subdivided into 11 subdomains. Each of the 11 subdomains contain specific residues and motifs or patterns of amino acids that are characteristic of that subdomain and are highly conserved. (Hardie, G. and Hanks, S. (1995) The Protein Kinase Facts Books, Vol I:7-20 Academic Press, San Diego, Calif.) In particular, two protein kinase signature sequences have been identified in the kinase domain, the first containing an active site lysine residue involved in ATP binding, and the second containing an aspartate residue important for catalytic activity.
The second messenger dependent protein kinases primarily mediate the effects of second messengers such as cyclic AMP (cAMP), cyclic GMP, inositol triphosphate, phosphatidylinositol, 3,4,5-triphosphate, cyclic ADPribose, arachidonic acid, diacylglycerol, and calcium-calmodulin. The cyclic-AMP dependent protein kinases (PKA) are important members of the STK family. Cyclic-AMP is an intracellular mediator of hormone action in all procaryotic and animal cells that have been studied. Such hormone-induced cellular responses include thyroid hormone secretion, cortisol secretion, progesterone secretion, glycogen breakdown, bone resorption, and regulation of heart rate and force of heart muscle contraction. PKA is found in all animal cells and is thought to account for the effects of cyclic-AMP in most of these cells. Altered PKA expression is implicated in a variety of disorders and diseases including cancer, thyroid disorders, diabetes, atherosclerosis, and cardiovascular disease. (Isselbacher, K. J. et al. (1994) Harrison's Principles of Internal Medicine, McGraw-Hill, New York, N.Y., pp. 416-431, 1887.)
PTKs specifically phosphorylate tyrosine residues on their target proteins and may be divided into transmembrane, receptor PTKs and nontransmembrane, non-receptor PTKs. Transmembrane protein-tyrosine kinases are receptors for most growth factors. Binding of growth factor to the receptor activates the transfer of a phosphate group from ATP to selected tyrosine side chains of the receptor and other specific proteins. Growth factors (GF) associated with receptor PTKs include epidermal GF, platelet-derived GF, fibroblast GF, hepatocyte GF, insulin and insulin-like GFs, nerve GF, vascular endothelial GF, and macrophage colony stimulating factor.
Non-receptor PTKs lack transmembrane regions and form complexes with the intracellular regions of cell surface receptors. Such receptors that function through non-receptor PTKs include those for cytokines, hormones, such as growth hormone and prolactin, and antigen-specific receptors on T and B lymphocytes.
Many of these PTKs were first identified as the products of mutant oncogenes in cancer cells where their activation was no longer subject to normal cellular controls. About one third of the known oncogenes encode PTKs, and it is known that cellular transformation (oncogenesis) is often accompanied by increased tyrosine phosphorylation activity. (Charbonneau H. and Tonks N. K. (1992) Annu. Rev. Cell Biol. 8:463-93.) Regulation of PTK activity may therefore be an important strategy in controlling some types of cancer.
Nek1 is an example of a dual specificity protein kinase from mouse capable of phosphorylating serine, threonine, and tyrosine residues. (Letwin, K. et al. (1992) EMBO J 11:3521-3531.) Nek1 contains an N-terminal kinase domain similar to the catalytic domain of NIMA, a serine/threonine protein kinase which regulates the cell cycle in the fungus Aspergillus nidulans. Nek1, however, is able to phosphorylate exogenous substrates on tyrosine as well as serine and threonine when expressed in bacteria. Nek1 is expressed at high levels in both male and female germ cells, consistent with a role in meiosis.
The discovery of a new Nek1-related protein kinase and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, treatment, and prevention of cancer and immune and reproductive disorders.